42 ROOT HABITS OF DESERT PLANTS. 



of air of the two seasons is mainly the operating cause in bringing about the 

 difference in the root-systems. The leading factors in this process would be 

 as follows. In summer the temperatures both of soil and of air permit, on 

 the one hand, very rapid growth, and on the other the best conditions for 

 water-absorption, with the result that the root-system of a plant is well 

 developed. In winter, on the other hand, the soil at the 20 to 30 cm. depth 

 is in daytime colder than the air, which operates directly to depress the rate 

 of water-absorption, to limit the development of the roots, and thus to make 

 the conditions unfavorable for the fullest growth of the shoot, with the 

 result that in turn the demands on the root-system are relatively low. 

 Could the winter soil be warmer than the air, the growth of the shoot would 

 probably be much more vigorous and the root-systems of the plant much 

 more extensive and of a different character than they actually are. 



From the foregoing brief statement of the leading general differences 

 between the annuals of summer and those of winter, it will appear that the 

 summer season may be conceived as favoring hygrophily more than the 

 winter season ; so that the root-systems of the summer annuals can be said 

 to tend toward hygrophily and those of the winter forms toward xerophily. 

 With these differences in mind, it is of interest to classify those of the 

 winter annuals which have a generalized type of root-system, or one which 

 approaches the type of the summer plants. As a fact, we find that Bowlesia 

 Parietaria, and Malva, the only plants examined with a marked general- 

 ized type of root-system, are limited in their normal distribution to situa- 

 tions where they enjoy relatively favorable water relations. The winter 

 annuals with specialized or winter type of root-system, that is, deeply 

 penetrating, have the widest distribution. 



Although no attempt has been made in this study to group plants accord- 

 ing to the various types of root-systems, or to describe the roots from the 

 standpoint of classification, such, for example, as has been done by Freiden- 

 feldt (Studien iiber die wurzeln krautiger Pflanzen. I. Ueber die Formbild- 

 ung der Wurzel vom biologischen Gesichtspunkte, Flora, 91, 1902), it has 

 been noted that each species examined had its peculiar form of roots. So 

 striking is the individuality of the root-systems that it may be possible, and 

 probably would be possible, to determine the form solely from the character 

 of its root. 



The most striking roots of autotrophic annuals examined were those of 

 Aster ianacetifolius, although those of Amsinckia and its relatives were also 

 very individual. The root-system of the parasite Orthocarpus purpurascens, 

 which was found in connection with 18 hosts, was more diversified than that 

 of any other annual. The roots were either coarse or fibrous, with a single 

 main root, or broken up into several main roots. Whatever may have been 

 the determining cause of this diversity, it was observed that the roots of the 

 parasite reflected in a degree the characters of the roots of the host plants. 



